A major goal of any dielectric deposition system is good step-coverage. Step-coverage refers to the ability of subsequent layers to evenly cover levels ("steps") already present on the substrate.
A problem arises when material from a point source is shadowed by "steps" on the substrate surface. The result can be that sections of the deposited layer can be too thin or have a void, i.e., "keyhole." A "keyhole" is a profile which is characterized by the presence of an unwanted void in a layer of deposited material, as seen in FIG. 1.
These effects are aggravated if the slope of the underlying profile is retrograde. Deposition of a nonconformal material onto a feature having a retrograde profile frequently results in a re-entrant profile, often accompanied by a "keyhole." As cell dimension sizes get smaller, and features are placed closer and closer together, the problem of "keyhole" formation will become greater.
The problem is also aggravated by an increase in aspect ratio (height to width ratio) of the feature. As the aspect ratio increases beyond 3:1, the profile becomes more retrograde, and the step coverage of the subsequently deposited film degrades.
Reactive ion sputter etching has been found to have a characteristic etch pattern. Sputter etch yields display angular dependency. This etch pattern makes reactive ion sputter etching a useful tool in forming faceted edges. The graph in FIG. 2 illustrates the Etch Angle versus Yield of the Etch. From the figure, it should be apparent that the Yield is highest at an angle of approximately 45.degree.. FIG. 3 illustrates the facet which results at the edges of a feature formed during the etch process. The facet angle is also approximately 45.degree..
In sputter etch, ions which impinge on horizontal surfaces have a minimal effect on etch rate and profile. However, the sputter yield of the etch at the corners is approximately four times that of the etch rate of a horizontal surface, thereby creating an extreme etch profile. The effect is the wearing, away of the corners of a feature at approximately 45.degree. angles. The material removed by the sputter etch is redeposited along the sides of the feature and along the surface of the substrate.
The amount of material redeposited is also effected by the actual shape of the feature being etched. Prograde and retrograde profiles tend to etch differently.
There are problems associated with the sputter of a conductive material. Material sputtered from the top corners of several conductive features may redeposit too far along the substrate between the features. If this material is allowed to remain conductive, an unwanted electrical short can be created between the features. Consequently, the tendency in the industry is to avoid sputter etching of conductive materials.
However, sputter etching is used extensively in planarization; e.g., in a bias sputter quartz (BSQ) process involving deposition/etch/deposition technology.
Known processes for depositing dielectrics use the sequence of an oxide deposition step, followed by facet etching of the deposited oxide, and another deposition step. This sequence involves the introduction of three additional processing steps, and the associated increases in manufacturing costs. The first oxide deposition further increases the aspect ratio of the feature.
Furthermore, known methods employing the sequence of an oxide deposition step, followed by a dry etch step, do not provide any significant degree of slope angle creation, and therefore can not reduce the aspect ratio by any significant amount. Hence, the known methods can not provide the step-coverage necessary for effective filling of mid-submicron features.